1. Field of the Invention
This invention relates to a process for preparing ultra-high molecular weight polyethylene (UHMWPE) multi-filament yarns having improved tensile properties and the yarns and articles thereby produced.
2. Description of the Related Art
UHMWPE multi-filament yarns have been produced possessing high tensile properties such as tenacity, tensile modulus and energy-to-break. The yarns are useful in applications requiring impact absorption and ballistic resistance such as body armor, helmets, breast plates, helicopter seats, spall shields; composite sports equipment such as kayaks, canoes bicycles and boats; and in fishing line, sails, ropes, sutures and fabrics.
Multi-filament “gel spun” ultra-high molecular weight polyethylene (UHMWPE) yarns are produced by Honeywell International Inc. DSM N.V. Toyobo Co. Ltd. and Tongyizhong Specialty Fibre Technology and Development Co., Ltd. The gel-spinning process discourages the formation of folded chain molecular structures and favors formation of extended chain structures that more efficiently transmit tensile loads.
The first description of the preparation and drawing of single UHMWPE filaments in the gel state was by P. Smith, P. J. Lemstra, B. Kalb and A. J. Pennings, Poly. Bull., 1, 731 (1979). Single filaments of UHMWPE were spun from solution and drawn while evaporating the solvent. More recent processes (see, e.g., U.S. Pat. Nos. 4,551,296, 4,663,101, 6,448,659 and 6,969,553 describe drawing all three of the solution filaments, gel filaments and the solvent-free filaments. The disclosures of U.S. Pat. Nos. 4,551,296, 4,663,101, 5,741,451, 6,448,659, and 6,969,553 and United States Application 20050093200 are hereby incorporated by reference to the extent not incompatible herewith.
The theoretical strength of a polyethylene molecule is directly related to its molecular weight. (D. C. Prevorsek, Handbook of Fiber Science and Technology, Vol. 3, Section 3.2, P. 48-59, Marcel Dekker. Inc., New York 1996. Y. Tremonia et al. Macromolecules, 18, 2246 (1985)). The experimentally realizable strength of a polyethylene fiber was found to be directly related to the molecular weight of the polyethylene from which the fiber is spun, and also related to the breadth of the molecular weight distribution of that starting polymer (P. Smith et al., J. Poly. Sci., Poly. Phys. Ed., 20 2229 (1982)). Higher fiber strength was favored by spinning polymers having weight average molecular weight to number average molecular weight ratios (Mw/Mn) less than seven. However, it appeared that the variation of Mw/Mn from 7 to 15.6 did not affect tensile strength.
U.S. Pat. No. 4,436,659 taught the spinning of UHMWPE having Mw/Mn lower than 5, the polymer having been obtained by fractionation of a polymer having a broader molecular weight distribution or by polymerization using specific catalyst systems and/or specific reaction conditions.
U.S. Pat. No. 5,547,626 taught intentional degradation of intrinsic viscosity (IV), polymer to fiber, to a final IV from 10% to 30% lower than the initial polymer IV. The upper bound on degradation was said to be necessary because an excessive decrease in the average molecular weight would cause a decrease in the fiber tenacity. The patent taught spinning of polyethylene solutions under oxidizing conditions, e.g., no use of antioxidant in the spinning solution and cooling of the extruded fibers in air. Among important process factors not specified in U.S. Pat. No. 5,547,626 were the extrudate temperature, the residence time in the single screw extruder, the intrinsic viscosity of the polymer in the extrudate, and the screw diameter, rotational speed and screw configuration.
A study by G. R. Rideal et al., titled, “The Thermal-Mechanical Degradation of High Density Polyethylene”, J. Poly. Sci., Symposium, No 37, 1-15 (1976) found that the presence of oxygen promoted shear induced chain scission, but that under nitrogen at temperatures less than 290° C., long chain branching and viscosity increase dominated.
A study by N. Dontula et al., titled “A Study of Degradation of High Density Polyethylene in a Corotating Intermeshing Twin Screw Extruder”. Poly. Eng. & Sci., 33 No, 5, 271-278 (1993) found similarly complex relationships between processing conditions and viscosity. Interaction between extruder temperature, screw speed and residence time caused directional changes in effects on viscosity.
Each of these references represented an advance in the state of the art, however none suggested the process or fibers of this invention, and none satisfied all of the needs met by this invention. In the process of the invention, it is believed that thermal-mechanical chain scission is as active as oxidative chain scission. The result is fibers with higher strength at lower intrinsic viscosity (lower molecular weight) than has previously been obtainable and composites with improved ballistic protection properties. A need has long existed for a multi-filament high strength polyethylene yarn having this combination of properties and a process for its production.